This invention relates to a shaft coupling for bottom cylinders of drafting units on spinning machines, in which one shaft has several shaft sections of different diameters which diminish toward the end in the area of one end. These shaft sections engage at one end of the other shaft in a longitudinal bore, which is adapted to the diameters by means of corresponding bore sections. One shaft section as well as a corresponding bore section has a screw thread for the form-locking transmission of the driving power, and the two other shaft sections with the corresponding bore sections are provided with fits for the forming of centering surfaces.
In the case of a shaft coupling of this type (European Patent Document EP 446 690 A1), one fit respectively forming the centering surfaces is provided on both sides of the screw thread. As a result, the guiding length of the centering surfaces as a whole is to be increased. The mentioned patent document contains nothing concerning the type of the fits for the centering surfaces so that it must be assumed that the two fits are narrowly tolerated clearance fits as they are normally provided for such shaft couplings. However, even if the tolerances are very narrow, a clearance fit at the centering surfaces always results in a certain radial deviation because the shaft sections on the centering surfaces tend to travel to the outside diameter of the bore section which is slightly larger in the bore.
It is an object of the invention to exclude radial deviations if at all possible in the case of a shaft coupling of the initially mentioned type.
This object is achieved in that one fit is a clearance fit and the other fit is a press fit.
As a result of the tolerance-caused overdimension of the assigned diameters of the press fit, the end areas of the shafts to be coupled are centered with respect to one another without any play. The clearance fit chosen for the second fit avoids redundancies in the area of the shaft coupling because a centering by means of an overdimension is carried out only at one of the two fits. The other fit has sufficient play even if this play in practice does not exceed 0.025 mm.
Advantageously, the clearance fit as well as the press fit are situated on the side of the screw thread facing away from the end of the shaft. This has the advantage that the centering surfaces of the longitudinal bore are situated as closely as possible at the end of the shaft containing the longitudinal bore, whereby the manufacturing precision becomes higher in comparison to a longitudinal bore which, according to the state of the art, has a centering surface in the area of the bore base.
In a further development of the invention, the threaded screw is first followed by the clearance fit and then by the press fit. As a result, the centering surface pertaining to the press fit is situated at the very start of the longitudinal bore which results in an easier mounting of the shaft coupling.
Expediently, the clearance fit is separated from the press fit by a conical shaft section. As a result, a gradual transition is created from the clearance fit to the press fit, in which case, at the same time, the threading is facilitated of the shaft sections of one shaft into the longitudinal bore of the other shaft.
In an advantageous development of the invention, a continuously identical diameter of the longitudinal bore is assigned to the clearance fit and to the press fit. This has the important advantage that, despite the two different fits, the longitudinal bore can be worked by means of a single tool adjustment in the area of the centering surfaces.
In an advantageous development of the invention, the shaft section assigned to the clearance fit is longer than the shaft section assigned to the press fit. In this case, the invention starts out from the recognition that the length of the shaft section assigned to the press fit should be as small as possible. This leads at the same time to reduced withdrawal forces during a demounting of the shaft coupling.
Expediently, the shaft section assigned to the clearance fit is at least twice as long as the shaft section assigned to the press fit. A length of the shaft section pertaining to the press fit of 3 mm and a length of the shaft section pertaining to the clearance fit of 7 mm were found useful.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.